By default, Docker works as an isolated single-node. All containers are only deployed onto the engine. Swarm Mode turns it into a multi-host cluster-aware engine.
Task: Initialise Swarm Mode
To use the secrets functionality, Docker has to be in "Swarm Mode". This is enabled via
docker swarm init
Join Swarm Mode
Execute the command below on the second host to add it as a worker to the cluster.
token=$(docker -H [[HOST_IP]]:2345 swarm join-token -q worker) && docker swarm join [[HOST_IP]]:2377 --token $token